Keyhole Markup Language (KML) is an XML notation for expressing geographic annotation and visualization within Internet-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Keyhole Earth Viewer. It was created by Keyhole, Inc, which was acquired by Google in 2004. KML became an international standard of the Open Geospatial Consortium in 2008. Google Earth was the first program able to view and graphically edit KML files. Other projects such as Marble have also started to develop KML support.
The KML file specifies a set of features (place marks, images, polygons, 3D models, textual descriptions, etc.) for display in Google Earth, Maps and Mobile, or any other geospatial software implementing the KML encoding. Each place always has a longitude and a latitude. Other data can make the view more specific, such as tilt, heading, altitude, which together define a "camera view" along with a timestamp or timespan. KML shares some of the same structural grammar as GML. Some KML information cannot be viewed in Google Maps or Mobile.
KML files are very often distributed in KMZ files, which are zipped files with a .kmz extension. These must be legacy (ZIP 2.0) compression compatible (i.e. stored or deflate method), otherwise the .kmz file might not uncompress in all geobrowsers. The contents of a KMZ file are a single root KML document (notionally "doc.kml") and optionally any overlays, images, icons, and COLLADA 3D models referenced in the KML including network-linked KML files. The root KML document by convention is a file named "doc.kml" at the root directory level but the first .kml file entry in the KMZ file is the actual one selected in Google Earth regardless of its name. By convention the root KML document is at root level and referenced files are in subdirectories (e.g. images for overlay images).
The MIME type associated with KML is application/vnd.google-earth.kml+xml; the MIME type associated with KMZ is application/vnd.google-earth.kmz.